What Thunderbolt 3 Means for PC Connectivity: An Explainer

Along with USB-C, Thunderbolt 3 shares a very compact, interchangeable port that delivers fast throughput rates. Here's what you need to know about connectivity on your next laptop.

As laptops continue to become thinner, shrinking to less than a half-inch in depth, PC manufacturers have found that current I/O ports like VGA, HDMI, and the 0.3-inch USB Type A can no longer fit into these sleeker notebooks. To replace them, the USB-C interface (also known as USB 3.1 or USB Type-C) was recently introduced in the 12-inch Apple MacBook and the latest Google Chromebook Pixel. USB-C is theoretically twice as fast as USB 3.0, and its form factor does away with up-and-down orientation (you can plug it any which way into the port). Now, Intel has announced the Thunderbolt 3 interface, which, thankfully, doesn't compete with USB-C, unlike the original Thunderbolt. Rather, it merges the strength of the faster Thunderbolt 3 interface with the potentially ubiquitous USB-C port.

What Is Thunderbolt 3? Thunderbolt 3 is the upcoming version of the Thunderbolt interface. The specifications have been released, but the interface itself is not available on PCs yet. Thunderbolt 3 lets you transfer data at up to 40Gbps. That's twice as fast as the 20Gbps throughput speed of Thunderbolt 2, and four times as fast as the 10Gbps of USB-C and the original Thunderbolt interface. Thunderbolt 3 lets you connect speedy hard drives, various displays (including 4K and 5K resolutions), and other peripherals, like PCIe Gen 3 expansion cages, to your desktop or laptop.

The big news, however, is the fact that Thunderbolt 3 is designed to work in the same-shaped port as USB-C and is compatible with USB-C cables and devices. The original Thunderbolt and Thunderbolt 2 interfaces use mini DisplayPort-shaped connectors, which mean that they are entirely incompatible with any USB port. Apple computers (and a few Windows workstations) are the only systems to support Thunderbolt, which ultimately led to limited adoption.

Sharing Ports Saves Space Older laptops with separate Thunderbolt 2 and USB 3.0 ports "waste" space, because when all is said and done, both ports do a similar job: They connect peripherals to the computer, albeit at different speeds.

Intel has built Thunderbolt 3 to work in the same exact physical ports as USB-C. That will help adoption, since PC manufacturers won't have to knock any additional holes in a system chassis for a separate Thunderbolt 3 port. It's important to note, however, that while they are physically the same, the Thunderbolt 3 port has circuitry behind it to allow faster Thunderbolt 3 throughput rates. Ports that only support USB-C (and not Thunderbolt 3) will likely be more common, even after Thunderbolt 3's official rollout later this year. To quickly check whether the port you have on a laptop is a USB-C or Thunderbolt 3 version, just look next to the port for a Thunderbolt icon, which resembles a lightning bolt.

What Happens When I Plug USB-C devices Into a Thunderbolt 3 Port? All USB-C devices can be plugged into, and will work in, a Thunderbolt 3 port, but it will transfer data at the slower USB-C speed. An easy thing to remember is that Thunderbolt 3 ports are technically backward-compatible with USB-C devices.

Thunderbolt 3, however, is not (necessarily) USB-C compatible. While it's true that you can physically plug a Thunderbolt 3 device into a USB-C port, it isn't guaranteed to work. Some Thunderbolt 3 devices, like power adapters, may charge your USB-C-only laptop, but devices that transfer data probably will not. You'll likely get a message on your laptop screen that the Thunderbolt 3 device is incompatible with the USB-C port.

Naturally, if you plug a Thunderbolt 3 device into a Thunderbolt 3 port, you'll get between 20 and 40Gbps maximum throughput, depending on the type of cable you're using.

Passive vs. Active Cables The simplest cables are passive and made of copper metal. Passive Thunderbolt 3 cables are identical to USB-C cables and plug into Thunderbolt 3 or USB-C ports. Thunderbolt 3 transfers data at up to 20Gbps over passive cables, at equivalent speeds to Thunderbolt 2, and double the speed of USB-C. But while passive cables are the most compatible with both Thunderbolt 3 and USB-C, they're not necessarily the most efficient.

To get all the benefits of Thunderbolt 3, you'll need to use active cables. Active Thunderbolt 3 cables will use integrated chips to achieve full 40Gbps throughput. You should use active cables where throughput really matters, like when connecting your laptop to 4K or 5K displays. You'll also want to use an active cable to get the fastest throughput out of local file storage for workstations and servers, particularly if you're connecting to a solid-state-drive (SSD)-based RAID array. Active cables made of copper are limited to about 6 feet in length–enough to go from your laptop to an external hard drive on a colleague's desk.

Active optical cables are another type entirely, made of plastic and glass, like fiber optic cables. They use optical (light) transmission to transmit the same 40Gbps throughput, but optical cables can extend their length from 6 feet up to roughly 200 feet. You're more likely to see optical cables used in long cable runs in a data center than on your desk. Optical cables will require switching over to optical ports, and aren't expected to ship until 2016.

As of this writing, there are no passive, active, or optical Thunderbolt 3 cables available. Intel says to expect them by the end of the year. Pricing has also not been announced, but we expect that a passive Thunderbolt 3 cables will have similar pricing to USB-C cables that are currently available (i.e., currently $10 to $25 for a 3-foot USB-C cable). Active Thunderbolt 3 cables will likely be more expensive, since they will have integrated circuitry, and will likely run from $30 to $50.

What About Adapters? Intel says that Thunderbolt 3-to-original-Thunderbolt adapters will be available at the end of this year, along with the first laptops with Thunderbolt 3 ports. You will be able to plug in current Thunderbolt devices, like hard drives, as well as mini DisplayPort and Thunderbolt displays (up to 5K), using this type of adapter, and the devices will run at the same speeds you're used to today.

A USB-C adapter will work when plugged into a Thunderbolt 3 port without any modification. That will give you access to HDMI displays, USB hard drives, Ethernet adapters, printers, scanners, and just about any other USB device. Of course, if you plug a USB 2.0 drive into a Thunderbolt 3 port with an adapter, data will transfer at a plodding 480Mbps throughput, but you can still access files on older drives this way.

Thunderbolt 3 is still in the nascent stages, and hasn't rolled out on any PCs yet, but since it shares some cables and its plug shape with USB-C, it has a much better chance for wide adoption than original Thunderbolt and Thunderbolt 2 had. Look for Thunderbolt 3 on premium and performance laptops later this year.

More Inside PCMag.com

About the Author

Joel Santo Domingo is the Lead Analyst for the Desktops team at PC Magazine Labs. He joined PC Magazine in 2000, after 7 years of IT work for companies large and small. His background includes managing mobile, desktop and network infrastructure on both the Macintosh and Windows platforms. Joel is proof that you can escape the retail grind: he wore a yellow polo shirt early in his tech career. Along the way Joel earned a BA in English Literature and an MBA in Information Technology from Rutgers University. He is responsible for overseeing PC Labs testing, as well as formulating new test methodologies for the PC Hardware team. Along with … See Full Bio